Various methods have been used for forming double wall corrugated pipe. The corrugations of the double wall pipe are initially formed from a first stream of plastic drawn into contact with mold blocks that define the shape of the corrugations. A second stream of plastic is subsequently extruded to form the inner wall of the pipe and merges with the inner edge of the corrugations. The double wall corrugated plastic pipe, due to the effective placement of material, can withstand significant axial loads and is commonly used for forming of plastic pipe in a diverse range of diameters. In addition to its high strength, the double wall pipe has the advantage that the inner wall of the pipe provides a smooth interior surface for the flow of liquid along the length of the pipe.
In order to obtain the desired strength in the molded plastic pipe, it is important to effectively center the extruder, and in particular, the extruder head relative to the circulating or moving mold blocks of a corrugator. This alignment determines tolerance variations in the thickness of the walls from one side of the pipe to the other side of the pipe. Improved and consistent strength is obtained when there is little or no variation in the pipe wall thickness about the circumference of the pipe.
Various approaches have been used to adjust the alignment of the extrusion head with the recirculating mold blocks. The combined extrusion head, extruder, and corrugator, are large and have a high mass. Therefore, aligning of the extruding equipment to the corrugator is difficult and somewhat sophisticated. The other approach is to adjust the mold blocks of the corrugator while keeping the extruder fixed. Depending upon the manufacture of the corrugator equipment and the type of mold blocks being used, this approach may be slightly more advantageous but it is still difficult and time consuming. Furthermore, poor alignment is expensive as excess plastic material is used to obtain the necessary strength. Excess plastic also contributes to slower manufacturing rates and/or higher costs.
In the manufacture of high strength plastic pipe such as double wall pipe, some ribbed pipe and changing thickness solid wall pipe, a cooling plug is associated with the extruder head and is designed to initially support the inner wall of the pipe and to also remove heat therefrom.
The present invention discloses an improved method and apparatus for the manufacture of such high strength pipe where problems associated with the mold blocks leading to excessive tolerance variations in the thickness of the walls of the pipe, are reduced.